Chemical kinetics

What is the rate of disappearance

Used if considering reactant concentration, rxn rate will be based on a decreace in conc. per unit time

What is the rate of appearance

Used if considering reactant concentration, rxn rate will be based on an increase in conc. per unit time

What is the formula for rate of appereance (A→B)

Δ[B]/Δt

What is the formula for rate of disappereance (A→B)

Δ[A]/Δt

What is the instantaneous rate

The rxn rate at one point in time

What are the four ways to monitor rates

1. Mass, volume, or pressure

• Useful when a rxn produces a gas

2. Change in pH

• Used to monitor rxns that produce of consume H+ OH- slowly over time

3. Change in conductivity

• Used for rxns where the total # of ions increases/decreases over time

4. Change in colour

• Useful when a rxn involves aq’s transition metal bc their ions are often colourful

What is the rate law expression

rate = k[A]^M [B]^n

How does the magnitude of k indicate the speed of the rxn

small k = slow rxn

large k = fast rxn

How can the magnitude of k increase

It increases w temperature

What would happen if the reactant is zero

• A zero-order reactant

• Changing the conc doesn’t affect the rxn rate

• Doesn’t participate in the rate-determining step of the rxn

What would happen if the reactant is one

• First-order rxn

• rate of rxn increases in a linear proportionality to the increase in reactant conc

• If [ ] doubles, rate doubles

What would happen if the reactant is two

• Second-order reactant

• Rate of rxn is exponentially related to rxn conc

• If [ ] doubles, rate quadruples

What two things does integrated rate law connect

concentration and time

What is the integrated rate law for first-order reactants

ln [A](t) = -kt + ln[A](o)

What is the integrated rate law for second-order reactants

1/[A](t) = kt + 1/[A](o)

What is the half-life equation for first order reactants

t(1/2) = -ln(1/2) / k

What is the half-life equation for second order reactants

t(1/2) = 1 / k[A](o)

What does the collision theory state

That particles must collide to react

What are the 2 criteria needed for a collision to be successful

1. Favourable orientation of the colliding particles

• Reactant bonds must break and the correct product bonds must form

  • The appropriate atom must come into contact so their electron densities can interact

2. Collisions must be energetic enough to surpass the activation energy barrier

What is activation energy

The minimum amount of energy that must be invested to form the activated complex/initiate a rxn

What is the transition state theory

Used to explain what happens as reactants ‘‘transition’’ into products, and can be illustrated using a potential energy diagram

What happens at a transition state

An activated complex forms

How do you find Ea (rev)

Ea (rev) = Ea (fwd) + l ΔH l

What is an activated complex

A temporary particle that forms as a result of a successful collision

What is each small step in a reaction pathway called

Elementary reaction

• Each step can’t be further simplified

What is important ab the stoichiometric coefficients in an elementary rxn

Each coefficient is equivalent to the rate law exponents

What is molecularity

How many particles are involved as reactants

What is it called when there is one, two, or three reacting particles

One:

Unimolecular

Two:

Bimolecular

Three:

Termolecular

Why are termolecular rxns unlikely

It is unlikely that 3 reacting particles will collide in correct orientation and energy

What 2 criteria must be met for a mechanism to be plausible

1. The steps in the mechanism must sum to give the balanced eq’n for the overall rxn

2. The rate law expression predicted by the rate-determining step in the mechanism needs to be the same as the experimentally determined rate law

When particles are formed in an early step and consumed in a later step

Intermediates

What is a catalyst

A substance that increases the rate of a rxn w/o itself being consumed

What are the two ways catalysts can work

1. They can hold the reactant particles in correct formation to let proper product bonds form

2. They can participate in the rxn and alter the rxn mechanism, resulting in a new rxn pathway w a lower activation energy

What’s the difference between a homogeneous and heterogeneous catalyst

A homogeneous catalyst is in the same state/phase as the reactant, while a heterogeneous catalyst is in a diff state/phase as the reactants

When particles are consumed in an early step and formed in a later step

Catalyst